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Rules for manipulating derivatives #121

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@mollygracehicks mollygracehicks commented Apr 16, 2025

New content

Resolves #30

Checklist

  • New pages are added to the table of contents.
  • Everything displays correctly in the web-generated preview.
  • Appropriate attribution has been given to source materials.
  • My name is included alphabetically in credits.md.

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github-actions bot commented Apr 16, 2025

PR Preview Action v1.6.1

🚀 View preview at
https://mphowardlab.github.io/essential-math/preview/pr-121/

Built to branch gh-pages at 2025-04-16 18:01 UTC.
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@mphoward mphoward requested a review from mzbush April 19, 2025 19:30
@mphoward mphoward changed the title Inserted notes Rules for manipulating derivatives Apr 19, 2025
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This is very good! The equations are all correct. I have some formatting suggestions to improve some of the organization, and some suggestions for where more explanations about the derivations is required.


---

#### Inversion
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@mzbush mzbush Apr 21, 2025

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Suggested change
#### Inversion
``` {topic} Inversion Rule

Putting the definitions of the rules for manipulating derivatives inside of {topic} directives will help them stand out. Do this for the chain rule as well.

Comment on lines +11 to +12
```{math}
\left( \frac{\partial x}{\partial y} \right)_z = \frac{1}{\left( \frac{\partial y}{\partial x} \right)_z}
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Suggested change
```{math}
\left( \frac{\partial x}{\partial y} \right)_z = \frac{1}{\left( \frac{\partial y}{\partial x} \right)_z}
\begin{equation}
\left( \frac{\partial x}{\partial y} \right)_z = \frac{1}{\left( \frac{\partial y}{\partial x} \right)_z}
\end{equation}

Wrapping your equations inside \begin{equation}\end{equation} for single equations and \begin{align}\end{align} for multiple equations is preferred over the {math} directory. This will automatically number the equations which is useful for if you want to reference them later. Do this for all your equations.

```

**Example:**

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In this example, it is not entirely clear what the goal of these derivations is. Adding a short explanation at the start to about what the example is trying to find/prove/demonstrate will make it easier for the reader to follow along.

The goal according to the original notes is to calculate $\left( \frac{\partial x}{\partial y} \right)_z$ and then show that calculating $\left( \frac{\partial y}{\partial x} \right)_z$ from $y = \pm \sqrt{xz}$ and then inverting will give the same result. Instead, it looks like the goal is to find $\left( \frac{\partial y}{\partial x} \right)_z$ without dealing with applying the power rule to a fractional power.

The issue is that the direct derivation of $\left( \frac{\partial x}{\partial y} \right)_z$ and the derivation starting from $y = \pm \sqrt{xz}$ are shown concurrently as if in sequence instead of as two separate paths that both find the same answer. Another problem is that calculating the derivative from $y = \pm \sqrt{xz}$ gets simplified too quickly. In the original notes, it starts as $\left( \frac{\partial y}{\partial x} \right)_z = \frac{1}{2}\sqrt{\frac{z}{x}}$ and then gets simplified to $\frac{z}{2y}$. This part is crucial to understanding how this partial derivative is being calculated.

\frac{1}{\left( \frac{\partial y}{\partial x} \right)_z} = \frac{2y}{z}
```

#### Chain Rule
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@mzbush mzbush Apr 21, 2025

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Should be in a {topic} directive

Comment on lines +44 to +46
```{math}
\left( \frac{\partial x}{\partial y} \right)_z = \left( \frac{\partial x}{\partial w} \right)_z \left( \frac{\partial w}{\partial y} \right)_z
```
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This equation could use some preamble to explain what the chain rule is and this new symbol $w$

\left( \frac{\partial x}{\partial y} \right)_z = \frac{1}{\left( \frac{\partial y}{\partial x} \right)_z}
```

**Example:**
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@mzbush mzbush Apr 21, 2025

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Wrapping examples in the {example} directive will make the page look better organized. Do this for the other example as well.

Comment on lines +56 to +59
Then:
```{math}
x = \frac{(w/z)^2}{z} = \frac{w^2}{z^3}
```
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Here, you bring back the definition of x from the earlier example. This is fine, but you should try to be explicit when an example is using a quantity defined in previous examples. You could do this by including the previously used definition of x in a preamble of this example that goes something like "Given {{blank}} and {{blank}}, find {{blank}}." This will ensure the reader doesn't need to go back to the previous example.

Since using \begin{equation}\end{equation} should number all of your equations, you may also want to be explicit about which equations are being combined to make the math more clear. You might rewrite this line as "Then, using the definition of x from Equation {{put equation number here}}:"

Comment on lines +72 to +74
```{math}
\left( \frac{\partial x}{\partial y} \right)_z = \frac{2w}{z^3} \cdot z = \frac{2w}{z^2} = \frac{2yz}{z^2} = \frac{2y}{z}
```
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This should be broken up into multiple lines using \begin{align}\end{align}

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Manipulating derivative
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